Torque coupling system

ABSTRACT

A torque coupling system includes an impeller that is connected to one end of an impeller shaft without the use of threads. The connection is made by cement and dowels. A drive system for the shaft includes a hollow coupling into which the other end of the shaft can be fitted. The shaft includes longitudinally extending slots formed at its end. The coupling includes spaced, longitudinally extending keys adapted to be fitted into the slots formed in the shaft. The drive system also includes a torque limiter that is connected intermediate the coupling and a drive motor. If the drive motor is an electric motor, the motor can be provided with automatic shut-down circuitry that is activated in the event of a current/torque overload. Visual and aural alarms are provided to alert the user that a shut-down has occurred.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to impeller shafts and, more particularly, to theconstruction of such shafts and to a drive system that (a) enables theshafts to be connected or disconnected quickly and (b) prevents theshafts from being fractured by excessive torque loads.

2. Description of the Prior Art

Impeller shafts are used in a variety of applications such as moltenmetal pumps, mixers, dispersers, and other equipment. In particular,impeller shafts made of graphite are used in environments that arehostile to shafts made of other materials. An especially desirable usefor graphite shafts is that of supporting rotatable impellers in moltenmetal pumps. Properly treated, graphite is an oxidation-resistantmaterial that enables the shafts to be effective in withstanding attackby corrosive substances such as molten aluminum.

A problem with graphite shafts is that they are not very strong.Consequently, it is difficult to work with them and they must be handledcarefully. In particular, it is relatively difficult to form threads onthe shafts. Another drawback of graphite shafts relates to the techniqueby which they are connected to drive motors and impellers. It isconventional to connect a graphite shaft to a drive motor by threadingthe shaft into a connector connected directly or indirectly to themotor. Unfortunately, the shaft cannot be adjusted axially relative tothe drive motor because the threaded end of the shaft is bottomed outupon connection to the drive motor. The same problem occurs when animpeller is threaded onto the other end of the shaft. Additionally,after the shaft has been used, torque that has been transmitted to theshaft during use will cause the shaft to be very tightly connected tothe drive motor and the impeller such that the components cannot beseparated easily. In extreme cases, it is necessary to destroy the shaftor the impeller in order to separate them from each other or to removethe shaft from the drive motor.

An additional problem related to graphite shafts is that they frequentlyare fractured in use due to excessive torque loads that are applied tothe shafts. In the particular instance of graphite shafts used in moltenmetal pumps, it sometimes happens that foreign objects are ingested intothe pumps. In this circumstance, an excessive torque load may be appliedto the shafts, resulting in catastrophic failure of the shafts.

Desirably, an impeller shaft would be available that would enable theimpeller to be easily connected to the shaft with minimal machining ofthe shaft and the impeller, and which would have superior strengthcharacteristics. Another advantageous feature would be a drivemotor-impeller shaft connection that would permit the shaft to beconnected and disconnected easily from the drive motor without requiringexcessive handling or machining of the shaft. Additionally, it would bedesirable to have a drive motor-impeller shaft connection that wouldprevent catastrophic shaft failure upon the intermittent application ofexcessive torque loads.

SUMMARY OF THE INVENTION

The present invention provides a new and improved torque coupling systemthat addresses the foregoing concerns. An impeller shaft according tothe invention is an elongate, cylindrical member that at one end isadapted to receive an impeller or other element, such as a rotor, and isadapted at its other end to be connected to a drive motor. The shaftrequires minimal machining, and it completely avoids the use of threads.The impeller-shaft connection is made by providing an opening throughthe center of the impeller and placing the shaft in the opening. Theimpeller is cemented to the end of the shaft to prevent axialseparation. Relative rotational movement is prevented by drilling axialholes into the shaft and the impeller at the intersection between theshaft and the impeller, and by thereafter placing dowels into the holes.The dowels serve as keys to prevent relative rotational movement betweenthe impeller and the shaft. At its other end, the shaft is provided withlongitudinally extending slots on its outer surface.

The drive system according to the invention includes a hollow couplinghaving first and second ends and defining a longitudinal axis ofrotation. The first end is adapted to be connected directly orindirectly to a drive motor and the second end is adapted to beconnected to the non-impeller end of the impeller shaft. The second endof the coupling includes a generally cylindrical opening into which theshaft can be fitted. A key is disposed within the opening in engagementwith the adapter, the key having a longitudinal axis that is alignedwith the longitudinal axis of the adapter. The adapter includescompression means for urging the key into contact with one of the slotsin the shaft, as well as retaining means for urging the key out ofcontact with the shaft. The retaining means is weaker than thecompression means such that the key can be brought into contact with theshaft but, upon releasing the compression means, the key will be movedout of contact with the shaft.

The invention also includes a torque limiter disposed intermediate theshaft and the drive motor. The torque limiter includes an input hubconnected to the motor, and an output hub connected to the coupling. Theinput and output hubs are connected by means of a friction disc and apressure plate. The disc and plate are spring-biased toward each otherin order to permit the input and output hubs to move relative to eachother whenever a predetermined torque load is exceeded. If the drivemotor is an electric motor, the control circuitry for the motor can beadapted such that power to the motor will be halted whenever apredetermined current/torque overload is attained. Visual and auralalarms will be activated to alert the user to the existence of ashut-down.

By use of the present invention, damage to the impeller shaft iseliminated or substantially reduced, in part because it is easy tomachine the longitudinally extending slots in the shaft, and in partbecause the torque limiter prevents catastrophic failure of the shaft.The impeller and the shaft can be connected easily and quickly, and theresultant connection is very strong. The particular construction of thecoupling enables the shaft to be connected and disconnected from thedrive motor quickly.

The foregoing and other features and advantages of the invention areillustrated in the accompanying drawings and are described in moredetail in the specification and claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, perspective view of a molten metal pump, showingan impeller shaft and drive system according to the invention;

FIG. 2 is a cross-sectional view of the impeller shaft and drive systemof the pump of FIG. 1;

FIG. 3 is an enlarged view of a portion of the impeller shaft and acoupling according to the invention;

FIG. 4 is a cross-sectional view of a portion of the impeller shaft andthe coupling according to the invention taken along a plane indicated byline 4--4 in FIG. 3;

FIG. 5 is a view similar to FIG. 4 taken along a plane indicated by line5--5 in FIG. 3;

FIG. 6 is a cross-sectional view of a torque limiter employed as part ofthe drive system according to the invention;

FIG. 7 is a bottom plan view of the impeller shaft according to theinvention;

FIG. 8 is a view similar to FIG. 7 showing an alternative embodiment ofthe impeller shaft according to the invention;

FIG. 9 is a cross-section view of the impeller shaft according to theinvention taken along a plane indicated by line 9--9 in FIG. 8; and

FIG. 10 is a schematic representation of electrical circuitry accordingto the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a molten metal pump 10 is indicated generally bythe reference numeral 10. The pump 10 is adapted to be immersed inmolten metal contained within a vessel 11. The vessel 11 can be anycontainer containing molten metal; in FIG. 1 the vessel 11 is theexternal well of a reverberatory furnace.

It is to be understood that the pump 10 can be any type of pump suitablefor pumping molten metal. Generally, however, and as particularly shownin FIG. 2, the pump 10 will have a base member 12 within which animpeller 13 is disposed. The impeller 13 includes a plurality ofradially extending openings 14. The impeller 13 is supported forrotation within the base member 12 by means of an elongate, rotatableshaft 15. The upper end of the shaft 15 is connected to a motor 16. Themotor 16 can be of any desired type.

The base member 12 includes a pumping chamber 17 and an outletpassageway 18 in fluid communication with the chamber 17. Because thepassageway 18 is disposed beneath the upper surface of the molten metal,the pump 10 functions as a so-called circulation pump, that is, itcirculates molten metal within the vessel 11. As indicated earlier,however, the pump 10 is described for illustrative purposes and it is tobe understood that the pump 10 can be of any type suitable for thepumping of molten metal.

A baffle plate 19 is connected to the upper portion of the base member12 and is spaced therefrom a small distance in order to define a fluidinlet 20. The baffle plate 19 is supported by a shaft bearing mount 21.A bearing ring 22 of silicon carbide or other material having bearingproperties at high temperature is disposed within the bearing mount 21.In like manner, a second bearing ring 23 of silicon carbide or othermaterial having bearing properties at high temperature is disposed atthe lowermost end of the base member 12 in facing relationship to thelowermost end of the impeller 13.

The shaft 15 typically is formed of graphite. It is to be understoodthat the present invention is especially effective with shafts made ofgraphite; however, the invention is usable with shafts made of othermaterials such as ceramic or coated metal. The use of the phrase"graphite shafts" herein is intended to encompass all such materials,where the use of such materials would be appropriate.

The first, or upper end of the shaft 15 is indicated by the referencenumeral 24. The second, or lower end of the shaft 15 is indicated by thereference numeral 26. The first end 24 is adapted to be connected to thedrive motor 16, while the second end 26 is adapted to be connected tothe impeller 13.

Referring particularly to FIGS. 7-9, the second end 26 is generallycylindrical, and is received within a cylindrical opening 28 formed inthe impeller 13. A cylindrical sleeve 30 is disposed about the lower end26. The upper end of the sleeve 30 engages a bearing ring 32 which isaxially fixed relative to the shaft 15. A bearing ring 33 is disposedabout the lowermost end of the impeller 13. The bearing rings 32, 33 aremade of silicon carbide or other material having bearing properties athigh temperature. The bearing rings 32, 33 in use are disposed in facingrelationship to the bearing rings 22, 23, respectively.

A second sleeve 34 is disposed about the shaft 15 at a vertical locationabove the ring 32. As is indicated in FIG. 9, the sleeve 30 not onlyserves to space the impeller 13 at a proper axial location relative tothe shaft 15, but it also serves to maintain the axial location of thebearing ring 32 relative to the base member 12. The sleeve 30 providesmechanical support for the bearing ring 32. In use, the bearing 32 issubjected to various stresses. The sleeve 30 helps to reduce prematurefailures of the bearing ring 32.

The impeller 13, the sleeves 30, 34, and the bearing ring 32 are securedto the second end 26 by means of refractory cement such as FRAXSETcommercially available from Metaullics Systems of Solon, Ohio. Therefractory cement prevents relative axial movement between the impeller13 and the shaft 15. In order to prevent relative rotational movementbetween the impeller 13 and the shaft 15, an opening 36 (FIG. 7) isformed in the impeller 13 and the shaft 1 at the interface between thetwo. The opening 36 is aligned with the longitudinal axis of the shaft15. If desired, a plurality of openings 36 (FIG. 8) can be provided.Dowels 38 (FIG. 9) are inserted into the openings 36 and retained thereby means of refractory cement. The dowels 38 thus function as keys.

As illustrated, the shaft 15 is cylindrical and the second end 26 isreceived within a cylindrical opening 28 by means of a non-threadedconnection. It will be appreciated that the second end 26 could takeother configurations such as splined, "square drive," and othernon-cylindrical forms. A cylindrical configuration as previouslydescribed is preferred, however, for a variety of reasons, including theready availability of cylindrical shafts and the simplicity and strengthof the previously described shaft-impeller connection.

Referring particularly to FIGS. 3, 4 and 5, the first end 24 includes apair of longitudinally extending slots 40. A hollow coupling 42 havingfirst and second ends and defining a longitudinal axis of rotation isdisposed about the first end 24. The first end of the coupling 42 isadapted to be connected indirectly to the drive motor 16 while thesecond end is adapted to be connected to the first end 24. A pair ofkeys 44 are disposed within the opening defined by the coupling 42 andare in engagement with the coupling 42. The keys 44 have longitudinalaxes that are aligned with the longitudinal axis of the coupling 42. Acompression means for urging the keys 44 into contact with the shaft 15is provided. The compression means is carried by the coupling 42 and, asillustrated, are in the form of set screws 46 that are threaded throughopenings formed in the side wall of the coupling 42.

A retaining means for urging the keys 44 out of contact with shaft 15also is provided. The retaining means is carried by the coupling 42 and,as illustrated, is in the form of a bolt 48 that is threaded into anopening formed in the keys 44 and through an unthreaded opening formedin the side wall of the coupling 42. A spring 50 is disposed about thebolt 48 and intermediate the head of the bolt 48 and the outer surfaceof the coupling 42. The spring 50 urges the bolt 48 radially outwardlyof the coupling 42. It will be apparent that the spring 50 is weakerthan the set screws 46 such that, upon actuation of the screws 46, thekeys 44 are brought into contact with the shaft 15 but, upon release ofthe screws 46, the keys 44 are brought out of contact with the shaft 15and retained in place relative to the coupling 42.

Referring particularly to FIGS. 2, 5, and 6, a torque limiter 60 isdisposed intermediate the motor 16 and the coupling 42. The torquelimiter 60 includes an input hub 62 connectable to a drive shaft 64 ofthe drive motor 16. An output hub 66 is connectable to the coupling 42by means of a block 68 through which a radially projecting drive pin 70extends. The pin 70 engages openings 71 formed in ears 72 included aspart of the output hub 66. A pair of shorter pins 73 (indicated by thedashed lines in FIGS. 3 and 5) extend out of the block 68 at rightangles to the pin 70. The pins 73 engages openings 74 formed in ears 75included as part of the coupling 42. The pins 70, 73 are supported bybronze bushings 76. Together, the block 68 and the pins 70, 73 form auniversal joint that permits the shaft 15 to flex relative to the outputhub 66.

The output hub 66 includes a radially extending, circumferential disc77. Friction-creating material 78 is disposed on each side of the disc77. The input hub 62 is in contact with the friction-creating material78 on one side of the disc 77, and a pressure plate 80 is in contactwith the friction-creating material 78 on the other side of the disc 77.The input hub 62 is connected to the plate 80 by means of bolts 81. Aspring 82 is disposed about each bolt 81 and is retained there by a snapring 83. The other end of the spring 82 bears against the upper surfaceof the input hub 62. By this technique, the springs 82 compress the hub62 and the plate 80 against the friction creating material 78, which inturn is compressed against the disc 77.

A retainer plate 84 is disposed about the hub 62 on that side of the hub62 closest to the motor 16. The plate 84 is connected to the plate 80 bymeans of spacer nuts 86, bolts 88, and torque pins 90. The torque pins90 extend through openings 91 formed in the hub 62 and openings 92formed in the pressure plate 80. The bolts 88 and the torque pins 90 arethreaded into the spacers 86. The torque pins 90 prevent the plate 80from rotating relative to the hub 62. A bearing 94 is disposedintermediate the central portions of the hubs 62, 66.

The torque limiter 60 has a number of desirable features. The number andstrength of the springs 82 can be selected so that variable torquelimits can be attained. It is expected that the springs can becolor-coded so that the user will be able to quickly identify thestrength of the springs, and the resultant torque limits of the torquelimiter 60. The friction-creating material 78 is exposed to view, andtherefore can be checked easily. Because the torque limiter 60 is boltedtogether, it is tamper-resistant and does not require torque adjustmentsduring installation or operation. The coupling 42 and the output hub 66have extremely low inertia, resulting in superior torsional overloadcontrol. In the event of an overload, the torque limiter 60 can bereused as soon as the jam has been cleared. Unlike shear pin-type torquelimiters, no resettings or parts replacements are needed to resumeoperation after an overload has been encountered; this is an importantconsideration in view of the high temperature environment in which thetorque limiter 60 must operate.

If the drive motor 16 is an electric motor, the control circuitry forthe motor 16 can be adapted easily to add a current sensor to sense whenthe motor 16 has been stalled. Referring particularly to FIG. 10, aportion of the electrical circuitry employed to operate the motor 16 isshown. The circuitry includes lead lines 100, 102, 104 that provide a460-volt, three-phase, 60 Hertz electrical current to an inverter drivepanel 106. Lead lines 108, 110, 112 are connected between the panel 106and the motor 16. An overload coil 114 is connected in series in each ofthe lead lines 108, 110, 112.

A torque limiter circuit 116 is powered by a control transformer 118that is connected across the lead lines 102, 104 by means of lead lines120, 122. The secondary side of the transformer 118 is connected to thecircuit 116 by means of lead lines 124, 126. A fuse 128 is connected inseries in the lead line 124. The circuit 116 includes a latchingcurrent-sensing relay 130 that is connected to the lead lines 124 bymeans of terminals 132, 134, respectively. The relay 130 is commerciallyavailable from Syrelec Corporation of Dallas, Tex., 75234, model numberLIRT 120AC. The relay 130 is a current-sensing relay that is energizedwhenever electrical current reaches a preselected value. The relay 130stays locked until reset. The relay 130 is adjustable both forhysteresis and threshold current activation.

The relay 130 includes normally closed contacts 136 and normally opencontacts 138. A lead line 140 connects the contacts 136, 138 and itselfis connected to the lead line 124 by means of a lead line 142. Thecontacts 136 are connected to a terminal 144 included as part of thepanel 106 by means of a lead line 146 and an overload relay 148 havingnormally closed contacts 150.

A visual alarm in the form of a lamp 152 is connected to the normallyopen contacts 138 by means of a lead line 154. An aural alarm in theform of a horn 154 is connected in parallel with the lamp 152 by meansof a lead line 156. A reset button 158 is connected to terminals 160,162 included as part of the relay 130 by means of a lead line 164. Acurrent transformer 166 is connected to terminals 168, 170 included aspart of the relay 130 by means of lead lines 172, 174.

In operation, a stall will occur whenever a particle of dross or otherforeign object (or build up of material) becomes trapped by the impeller13, and the torsional limit of the torque limiter 60 is exceeded. If thejam occurs quickly enough, it is possible that the torque limiter 60will enable the impeller 13 to power through the jam. If the jam occursmore slowly, the motor 16 probably will stall. Any time the torsionallimit of the torque limiter 60 is exceeded for more than a fraction of asecond, a current sensor included as part of the inverter drive panel106 will sense a sharp increase in current being drawn by the motor 16and will activate the relay 148 so as to discontinue motor rotationinstantly. Simultaneously, the current transformer 166 will activate acurrent sensor included as part of the relay 130 that is connectedacross the terminals 168, 170. Thereafter, the contacts 136 will openand the contacts 138 will close. In turn, the aural and visual alarms152, 154 will be activated indicating to the user that pump operationhas been discontinued automatically. Before restarting the system, thesource of the jam must be identified and cleared. The reset push button158 must be pushed to reset the relay 130, including the aural andvisual alarm contacts 136, 138. As noted previously, due to theparticular construction of the torque limiter 60, it is not necessary toreset any of the mechanical hardware.

Tests have been conducted with the components connected as illustratedin FIG. 2. A conventional threaded graphite shaft is able to sustainapproximately 150 foot-pounds of static torque before fracturing. By useof the present invention, a shaft of similar diameter can withstandapproximately 320 foot-pounds of static torque before fracturing.Because of the increased strength capability of the impeller shaft 15,the torque limit of the torque limiter 60 can be set at about 200foot-pounds. The enhanced strength capability of the drive systemaccording to the invention represents a significant improvement overconventional graphite shaft drive systems, and it enables the pump 10 tooperate for a much longer time in conditions that would have producedcatastrophic shaft failure with conventional impeller shafts and drivesystems.

Although the invention has been described in its preferred form with acertain degree of particularity, it will be understood that the presentdisclosure of the preferred embodiment has been made only by way ofexample and that various changes may be resorted to without departingfrom the true spirit and scope of the invention as hereinafter claimed.It is intended that the patent shall cover, by suitable expression inthe appended claims, whatever features of patentable novelty exist inthe invention disclosed.

What is claimed is:
 1. An impeller shaft assembly for connection to adrive motor, comprising:an elongate shaft having first and second ends,the first end adapted to be connected to the drive motor and the secondend adapted to receive an impeller, the second end of the shaft beinggenerally cylindrical; an impeller adapted to be connected to the secondend of the shaft, the impeller including a generally cylindrical openinginto which the second end of the shaft can be placed; longitudinallyextending openings formed in the impeller and the second end of theshaft at the interface between the second end and the impeller; dowelsdisposed within the openings formed in the impeller and the shaft; andrefractory cement disposed intermediate the second end of the shaft andthe impeller.
 2. The shaft assembly of claim 1, further comprising abearing ring disposed about the shaft intermediate the first and secondends, the ring being axially fixed relative to the shaft; anda sleevedisposed about the shaft adjacent the second end of the shaft, thesleeve being in contact with the ring at one end and the impeller at theother end.
 3. The shaft assembly of claim 1, wherein the first end isgenerally cylindrical, and includes a longitudinally extending slotformed on its outer surface.
 4. The shaft assembly of claim 1, furthercomprising a torque coupling system for the shaft, said torque couplingsystem including:a hollow coupling having first and second ends anddefining a longitudinal axis of rotation, the first end adapted to beconnected to the drive motor and the second end adapted to be connectedto the first end of the shaft, the second end defining a generallycylindrical opening into which the shaft can be fitted; a key disposedwithin the opening and in engagement with the coupling, the key having alongitudinal axis that is aligned with the longitudinal axis of thecoupling; compression means for urging the key into contact with theshaft, the compression means being carried by the coupling; andretaining means for urging the key out of contact with the shaft, theretaining means being carried by the coupling, the retaining means beingweaker then the means for urging whereby, upon actuation of thecompression means, the key is brought into contact with the shaft but,upon release of the compression means, the key is brought out of contactwith the shaft and retained in place relative to the coupling.
 5. Theshaft assembly of claim 1, further comprising a universal joint disposedintermediate the drive motor and the first end of the shaft.
 6. Theshaft assembly of claim 5, wherein the universal joint is in the form ofa block having first pins projecting from opposite sides thereof, thefirst pins being connected to a selected one of the motor or the firstend, and second pins projecting from opposite sides of the block, thesecond pins being connected to the other of the motor or the first end,the first and second pins being disposed at right angles to each other.7. An impeller shaft assembly for connection to a drive motor,comprising:an elongate shaft having first and second ends, the first endadapted to be connected to the drive motor and the second end adapted toreceive an impeller, the second end of the shaft being generallycylindrical; an impeller adapted to be connected to the second end ofthe shaft, the impeller including a generally cylindrical opening intowhich the second end of the shaft can be placed; longitudinallyextending openings formed in the impeller and the second end of theshaft at the interface between the second end and the impeller; dowelsdisposed within the openings formed in the impeller and the shaft; and atorque coupling system for the shaft, said torque coupling systemincluding: a hollow coupling having first and second ends and defining alongitudinal axis of rotation, the first end adapted to be connected tothe drive motor and the second end adapted to be connected to the firstend of the shaft, the second end defining a generally cylindricalopening into which the shaft can be fitted; a key disposed within theopening and in engagement with the coupling, the key having alongitudinal axis that is aligned with the longitudinal axis of thecoupling; compression means for urging the key into contact with theshaft, the compression means being carried by the coupling; andretaining means for urging the key out of contact with the shaft, theretaining means being carried by the coupling, the retaining means beingweaker then the means for urging whereby, upon actuation of thecompression means, the key is brought into contact with the shaft but,upon release of the compression means, the key is brought out of contactwith the shaft and retained in place relative to the coupling.
 8. Theshaft assembly of claim 7, further comprising refractory cement disposedintermediate the second end of the shaft and the impeller.
 9. The shaftassembly of claim 7, further comprising a bearing ring disposed aboutthe shaft intermediate the first and second ends, the ring being axiallyfixed relative to the shaft; anda sleeve disposed about the shaftadjacent the second end of the shaft, the sleeve being in contact withthe ring at one end and the impeller at the other end.
 10. The shaftassembly of claim 7, wherein the first end of the shaft is generallycylindrical, and includes a longitudinally extending slot formed on itsouter surface, the slot being engageable with the key disposed withinthe opening in the coupling.
 11. The shaft assembly of claim 7, furthercomprising a universal joint disposed intermediate the drive motor andthe first end of the coupling.
 12. The shaft assembly of claim 11,wherein the universal joint is in the form of a block having first pinsprojecting from opposite sides thereof, the first pins being connectedto a selected one of the motor or the first end of the coupling, andsecond pins projecting from opposite sides of the block, the second pinsbeing connected to the other of the motor or the first end of thecoupling, the first and second pins being disposed at right angles toeach other.